My main computer is experiencing heavy downloads (400kb/s), which I can't stop

Download and install the free version of Glasswire. This will at least tell you what is doing the downloading and how much. Those on a Classic plan try to defer the downloads until the LNFZ. If you are on a Liberty Plan, it doesn't much matter when the downloads occur.

I suspect your compute has picked up some sort of a virus, or spyware, and is downloading lots of data on you. The Glasswire JEP has suggested will tell you what is dowing the downloading. I love the info Glasswire shows.

Also make sure you're not observing this while viewing the modem status page because those bytes you see changing are just the modem refreshing your display and is not counted as actual download data in the Exede download totals but is causing the counter you see in the modem status display to change at about the rate you describe.